When printing sheets, such as paper sheets, in printing machines for securities and other similar security documents, it has to be avoided that sheets which are transferred from one machine to another machine are put in said other machine on the wrong side or wrongly orientated. Indeed, not all the printing operations are carried out in the same machine, therefore it happens that printed sheets are transferred from one machine to another or are even taken out of a machine in case of a problem in said machine. For this reason, it has to be ensured that printed sheets are always in a definite and known position with respect to the printing machine with their reference side properly aligned with the machine reference for the successive printings made on the sheet to be in register on the same side of the paper and also recto-verso.
Therefore, even if the sheets are transported manually or displaced by other means, one has to be sure that when introduced in a machine, they are in the proper position. If not, the machine must be stopped or the sheet in improper position must not be printed to avoid loss of paper sheets.
It is already known to apply printed markings onto the sheets, in particular in a non-printed area of the sheets (such as the margin or spaces between prints on the sheet), which markings are then read by appropriate reading units (see for instance U.S. Pat. No. 4,165,465 or U.S. Pat. No. 6,601,507).
Alternatively, it is known to provide the sheets with notches, cut-outs or embossed areas which are detected by appropriate detection means (see for instance U.S. Pat. No. 5,196,868 or GB 956 145).
Both solutions however require additional pre-processing of the sheets on the printing machines in order for the control markings to be applied thereon, such pre-processing being necessarily performed at the input of the first printing machine where the sheets are first processed. This accordingly implies hardware resources as well as the use of consumables for performing application of the control markings.
Further, the control markings must be applied in non-printed areas of the sheets (such as in the margins or between columns or rows of prints) if the control markings are not to interfere with the prints. This accordingly limits the possibilities to control the orientation and/or position of the sheets as well as the ability to dispose the necessary marking and reading stations on the printing machine.